1. Field of the Invention
The present invention relates to a frequency recovery apparatus and method for use in a digital broadcast receiver, and more particularly to a coarse frequency recovery apparatus and method which has very strong resistance to a sampling offset in a Digital Video Broadcasting-Terrestrial (DVB-T) broadcast receiver.
2. Discussion of the Related Art
Generally, a DVB-T broadcast system can perform data transmission/reception using an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
According to the basic principles of the above-mentioned OFDM scheme, a data stream having a high transfer rate is divided into a plurality of data streams, each of which has a low transfer rate, and the divided data streams are simultaneously transmitted using a plurality of sub-carriers.
The above-mentioned OFDM scheme has been widely used to transmit/receive broadcast signals due to a variety of advantages, the most important of which is that broadcast signals based on the OFDM scheme have very strong resistance to either frequency selective fading or narrowband interference.
A plurality of carrier systems can be efficiently implemented using an Inverse Fast Fourier Transform (IFFT) based on orthogonality among a plurality of carriers.
An OFDM receiver basically performs a carrier recovery operation to synchronize a desired signal. A frequency synchronization operation allows a Radio Frequency (RF) carrier frequency of a transmitter to coincide with that of a receiver.
The difference between carrier frequencies of the transmitter and the receiver is referred to as “frequency offset”. The OFDM scheme has very weak resistance to the frequency offset.
The frequency offset incurs two serious problems in a signal transmitted according to the OFDM scheme. One problem is that the magnitude of a signal transmitted via individual sub-carriers decreases when the signal is demodulated using a Fast Fourier Transform (FFT) process. The other problem is that orthogonality between sub-carriers is no longer maintained due to the occurrence of Inter-Carrier Interference (ICI).
In other words, the OFDM scheme has a relatively narrow frequency interval between sub-carriers as compared to a transmission band, such that the sub-carriers may be greatly affected by a small frequency offset.
Therefore, when transmitting/receiving a desired signal according to the OFDM scheme, the frequency synchronization technology can be considered to be one of important technologies capable of improving performance of the OFDM receiver.
The OFDM scheme controls the frequency synchronization action to be operated in two modes. The above-mentioned two modes are a coarse frequency synchronization mode and a fine frequency synchronization mode.
The coarse frequency synchronization mode estimates an integer multiple of a sub-carrier interval closest to an initial frequency offset, and compensates for the estimated result.
The fine frequency synchronization mode estimates a frequency offset lower than half of an interval between neighboring sub-carriers, and compensates for a frequency on the basis of the estimated frequency offset.
In other words, the coarse frequency synchronization method can allow an initial frequency offset of higher than the sub-carrier interval to be reduced to half of the sub-carrier interval or less. The frequency offset reduced to half of the sub-carrier interval or less can be compensated for using the fine frequency synchronization method.
As described above, the coarse frequency synchronization method must estimate an integer multiple of the sub-carrier interval closest to the initial frequency offset. A basic configuration of the above-mentioned coarse frequency offset estimator is shown in FIG. 1.
FIG. 1 is a block diagram illustrating a conventional coarse frequency offset estimator.
The coarse frequency estimator shown in FIG. 1 performs correlation between FFT output signals and the delayed results of the FFT output signals, such that it determines a changed value of a predetermined pilot pattern, at which the maximum corresponding value is generated, to be a coarse frequency offset.
In order to determine the above-mentioned coarse frequency offset, a pilot pattern generator 121 generates 45 pilot patterns at a 2k mode, and generates 175 pilot patterns at an 8k mode. The pilot pattern generator 121 generates the remaining values other than the pilot patterns with padding values of zeros at the 2k mode or at the 8k mode.
Using continual pilot informations generated from the pilot pattern generator 121, current symbols generated from the FFT unit 107, and one-symbol-delayed symbols generated from the delay 117 are correlated in a cross-correlator 119, such that their correlation results are generated from the cross-correlator 119.
An estimator 115 receives output values of the correlator 119, tracks a maximum value from among a plurality of correlation values contained in a data interval, and estimates a coarse frequency offset using position information of the maximum value.
The above-mentioned frequency offset is generated because a frequency of a received signal passing through the FFT unit 107 moves on a frequency domain. If there is no frequency offset, the maximum value from among the correlation values is generated at a specific position at which a data interval is terminated.
Therefore, the coarse frequency offset can be estimated according to variations of the maximum value position.
An accumulator 113 shown in FIG. 1 compensates for the coarse frequency offset acquired by the above-mentioned estimation process.
If a sampling clock offset is generated before the above-mentioned coarse frequency offset estimation method, and the generated sampling clock offset passes through the FFT unit 107, the FFT unit 107 converts the sampling clock offset into a phase variation value, such that the phase variation value has a negative influence upon the coarse frequency offset estimation result.
In other words, generally, if the sampling clock offset is 100 ppm, a single sample is inserted or omitted at intervals of a predetermined distance corresponding to four symbols in the 2k mode. In the case of the 8k mode, a single sample is inserted or omitted at intervals of a predetermined distance corresponding to a single symbol.
Therefore, a frequency offset is slightly robust to the sampling clock offset at the 2k mode, but it has very weak resistance to the sampling clock offset at the 2k mode.
In other words, the apparatus of the above-mentioned conventional coarse frequency offset estimation and the method thereof has more difficulty in estimating the frequency offset in the 8k mode than in the 2k mode under a predetermined sampling clock offset environment (e.g., if the sampling clock offset is 100 ppm).